Original author: Jaleel Jialiu, BlockBeats

Earlier this year, when I wrote "13 lines of code to help Bitcoin realize smart contracts? Understanding OP_CAT soft fork", OP_CAT was still an unfamiliar term to many people. If it weren't for the Bitcoin NFT project Taproot Wizards using meme culture to launch the Quantum Cat series NFT to promote OP_CAT, few people would know this seemingly boring technical concept.

But just half a year later, OP_CAT has become very popular and is even regarded as the next "Lightning Network". The recently popular Fractal Bitcoin is actually the code implementation of OP_CAT in Bitcoin. The token FB tripled or quadrupled in just a few days after it went online. Therefore, in the Bitcoin ecosystem, multiple OP_CAT-based protocols were born in just one day, such as CAT 20 on the Fractal Network, which also focuses on the concept of OP_CAT. In the days of casting, the GAS of the Fractal Network was once pulled to more than 5,000. At present, the price of a CAT in the over-the-counter market is around $5.5, and there are even very few orders without a market. Early minters have achieved an increase of 5 to 20 times. And Quantum Cat has been able to maintain a price of 0.25 BTC, becoming a blue chip of Bitcoin NFT.

It seems that as long as it touches OP_CAT, this thing can pull the plate.

OP_CAT is not only a "self-congratulation" of the community, but also a technology that is discussed a lot among Bitcoin developers. I looked through this year's Bitcoin Optech newsletter and Bitcoin developer conferences, and OP_CAT frequently appeared in discussions, and it is a regular.

Currently, the OP_CAT Bitcoin BIP draft jointly released by Bitcoin Core developers Ethan Heilman and Armin Sabouri has now been officially named BIP 347. Many heavyweights in the Bitcoin field, such as Tadge Dryja, the author of the Lightning Network white paper, Olaoluwa Osuntokun, CTO of Lightning Labs, the main developer of the Lightning Network, and Andrew Poelstra, director of research at Blockstream, have shown great support for OP_CAT.

Why do Lightning Network developers favor OP_CAT?

To understand this phenomenon, we have to start with the Lightning Network. In 2011, Satoshi Nakamoto mentioned the prototype of the Lightning Network in an email. As a key solution for Bitcoin expansion and payment speed improvement, the Lightning Network has attracted the attention of a large number of developers since its inception.

In the past two years, the concept of "Bitcoin ecology" has gradually gained momentum, and various Bitcoin expansion solutions such as side chains and virtual machines have sprung up, attracting many investors, but in the eyes of those senior Bitcoin developers, they are not mainstream and are not worthy of being accepted by the public. After all, the Lightning Network is currently the best "payment channel" in Bitcoin in the eyes of the public, and it is also one of the few "posthumous children" of Satoshi Nakamoto and the "son of legitimacy" in the spirit of the Bitcoin community.

However, things have changed dramatically in the past year. Several developers have announced their withdrawal from work related to the Lightning Network, and voices denying the Lightning Network have been heard one after another, especially from some senior developers. Fiatjaf, the founder of Nostr, was even more blunt: "The Lightning Network has been defrauding Bitcoin users of their time, energy, and money for six years."

In this context, some developers are looking for the next "Lightning Network". OP_CAT is considered by many Bitcoin developers to be the next major breakthrough for Bitcoin after the Lightning Network. Since OP_CAT is highly technical, before discussing what OP_CAT is and what OP_CAT can do, let's first get to know the Bitcoin developers who favor OP_CAT.

Bitcoin developers supporting OP_CAT

Lightning Network White Paper Author

Tadge Dryja is one of the authors of the Lightning Network white paper. In 2015, Tadge Dryja and Joseph Poon, another author of the Lightning Network white paper, founded Lightning Labs under the leadership of Elizabeth Stark. It stands on equal footing with another Bitcoin Lightning Network company BlockStream. The biggest difference between the two is that Lightning Labs uses the Go programming language, while Blockstream uses the C programming language.

However, at the end of 2016, Tadge Dryja had disagreements and quarrels with the Lightning Labs team, so when Lightning Labs was just one year old, Tadge Dryja chose to leave and join the Digital Currency Initiative Community (DCI) at MIT to continue his research on the Lightning Network. At MIT DCI, he participated in multiple research projects, mainly focusing on the scalability and interoperability of cryptocurrencies and smart contracts. This included the development of the Utreexo project, a new Bitcoin scalability technology designed to make Bitcoin nodes smaller and faster. In 2022, Tadge Dryja joined Lightspark as a senior research scientist. At Lightspark, he continued to solve the scalability problems of Bitcoin and blockchain, using his expertise as a co-inventor of the Lightning Network.

It is precisely because of his profound attainments in Bitcoin smart contracts and scalability technology that Tadge Dryja discovered the potential of OP_CAT early on, and has always supported testing OP_CAT on the Bitcoin test network and encouraged developers to try to "destroy" OP_CAT to discover its potential problems.

Lightning Labs CTO

Olaoluwa Osuntokun (Roasbeef) is another co-founder and CTO of Lightning Labs. He is also an outstanding Bitcoin developer who has made important contributions to the research and development of the Lightning Network. It can be said that the early team of Lightning Labs is not inferior to Blockstream.

Speaking of Olaoluwa Osuntokun, we have to mention the "gentleman's dispute" between him and Tadge Dryja. Interestingly, Olaoluwa was a big reason why Tadge Dryja left Lightning Labs. When Tadge Dryja worked at Lightning Labs, the first version of the protocol he developed was called LIT, which was not compatible with BOLT developed by Blockstream, but Olaoluwa's was compatible, so it gradually gained recognition and favor from more developers, completing the latecomer's success, which to some extent accelerated Dryja's departure.

However, history is always full of drama. Today, these two “old enemies” from Lightning Labs are now jointly supporting OP_CAT.

Head of Research, Blockstream

There seems to be a rumor among the people that Blockstream is the "boss" behind Bitcoin. Such rumors are not groundless. In 2014, Adam Back, the "father of PoW", gathered many well-known early Bitcoin developers such as Matt Corallo, Greg Maxwell, and Pieter Wuille to establish Blockstream, and in the subsequent Bitcoin expansion war, he clearly stood up and promoted the Lightning Network, which has made the Lightning Network what it is today.

At present, in the discussion about OP_CAT, Andrew Poelstra, the head of research at Blockstream, is also a person who can never be avoided. Andrew Poelstra is the director of research at Blockstream and a senior Bitcoin cryptography scripting developer. His influence in the industry is self-evident. As early as January 30, 2021, he wrote an article titled "CAT and Schnorr Tricks I" to discuss OP_CAT, pointing out that combining OP_CAT with CHECKSIGFROMSTACK can provide a clever method of transaction introspection.

Although Blockstream has not yet created a separate blockchain for OP_CAT, they have provided support for testing OP_CAT through Bitcoin Inquisition, a test network used to evaluate new Bitcoin features. This is like building a safe "training ground" for OP_CAT, allowing researchers to carefully observe its performance under real-world conditions.

Andrew Poelstra also compared OP_CAT to other proposals such as OP_CTV, pointing out that OP_CAT provides broader flexibility while OP_CTV is more narrowly focused on non-recursive smart contracts.

Author of BIP 347

In the promotion of OP_CAT, Bitcoin developers Ethan Heilman and Armin Sabouri made a significant contribution. They co-authored and promoted the proposal of OP_CAT, which has been formally designated as BIP 347. This proposal aims to redefine the opcode OP_SUCCESS 126 through a soft fork in order to re-enable OP_CAT.

Armin Sabouri has focused on improvements to the Bitcoin script and consensus layers in the past. He believes that although OP_CAT is not the final solution, it provides the possibility of implementing various new functions and is an important step in improving Bitcoin scripts.

Ethan Heilman is also the person who influenced Andrew Poelstra's thinking. It was during a private communication in the fall of 2019 that they changed the minds of those who did not support the so-called Bitcoin smart contract function of Andrew Poelstra. Ethan Heilman pointed out that although people have concerns about the so-called Bitcoin smart contract function, these smart contracts that are considered harmful can actually be implemented through CHECKMULTISIG. To prove this, Ethan Heilman launched a challenge on social media, encouraging people to come up with feasible "dark" smart contracts, but no one has succeeded so far.

StarkWare

StarkWare is a company focused on developing blockchain expansion technology, and is particularly good at using zero-knowledge proof (ZKPs) technology to enhance the privacy and efficiency of transactions. The company is committed to outsourcing complex computations from main chains (such as Ethereum) to secondary networks without sacrificing security and decentralization. In this way, StarkWare solves the scalability problem of blockchain while maintaining the transparency and verifiability of transactions.

StarkWare's promotion of OP_CAT stems from its interest in Bitcoin network expansion and smart contract functionality. OP_CAT is a Bitcoin opcode that allows more complex transactions and contracts to be executed on the Bitcoin network through the concatenation of data. By enabling OP_CAT, StarkWare attempts to implement smart contract capabilities similar to Ethereum on Bitcoin, thereby expanding Bitcoin's use cases and enhancing its functionality.

In July 2022, StarkWare launched a $1 million research fund specifically for studying the pros and cons of enabling OP_CAT on Bitcoin. This fund is intended to fund researchers and developers who have insights into how to implement OP_CAT securely and efficiently on Bitcoin.

In addition, StarkWare has demonstrated the potential of implementing zero-knowledge proofs using OP_CAT in Bitcoin’s test environment, specifically on Signet. These efforts demonstrate how OP_CAT can be used to implement more complex operations on the Bitcoin network, such as the application of zero-knowledge proofs, which are a key part of smart contract functionality. Recently, StarkWare has also established a partnership with sCrypt to explore the design of a PoC (proof of concept) bridge built on OP_CAT and ZK magic, which shows that StarkWare may also launch an OP_CAT-related protocol in the near future.

other

The other developers are not so famous, so we will talk about them together.

Salvatore Ingala has been deeply studying Bitcoin payment pools and smart contracts. He proposed a solution to use OP_CAT to optimize the payment pool exit process, believing that this can significantly reduce on-chain data and operation costs. Ingala sees OP_CAT as a potential plug-in for other Bitcoin scaling strategies, such as Arc and Coinpools, or even future Optimistic rollups for Bitcoin to increase efficiency and security.

Anthony Towns is one of the main developers of Bitcoin Inquisition, a tool similar to the test network but more flexible for testing Bitcoin protocol changes that have not yet been widely accepted. Towns promoted the activation of OP_CAT on the Inquisition platform, providing a safe "testing ground" for OP_CAT. Although Towns recognizes the importance of OP_CAT in testing new features and exploring the capabilities of Bitcoin scripts, he is still cautious about adding too much programmability to Bitcoin, fearing that this may increase the risk of Bitcoin being censored or controlled.

Robin Linus, the creator of BitVM, believes that the reintroduction of OP_CAT provides a powerful tool for Bitcoin, especially to support projects like BitVM, which make it easier and more efficient to verify arbitrary computations on Bitcoin. With the implementation of OP_CAT, the Bitcoin ecosystem is able to create more general and expressive smart contracts, promoting a more functional and practical blockchain environment.

Why is there so much attention being paid to the development of the Lightning Network?

Why do Lightning Network developers show such great interest in OP_CAT? To understand this, we need to first understand the essence of OP_CAT.

What is OP_CAT?

OP_CAT is an opcode in Bitcoin script that is used to concatenate two data fragments on the stack into a larger element. Although its function seems simple, it can provide more smart contract capabilities for the Bitcoin network, allowing developers to create and process more complex data and transaction logic.

OP_CAT is not a new opcode. It existed in early versions of Bitcoin, but was disabled in later versions due to security and complexity concerns. However, as Bitcoin continues to develop, more and more developers believe that it is time to re-enable OP_CAT.

But why does this seemingly simple operation spark so much discussion among Bitcoin developers? One of the key reasons is that it can significantly improve the smart contract capabilities of the Bitcoin network, allowing developers to implement some functions that are more complex or even impossible to implement in current Bitcoin scripts.

OP_CAT’s potential in payment applications

OP_CAT has shown great potential for application in payment systems, especially in off-chain protocols and payment channel networks. Its re-enabling will greatly improve the operational efficiency of these systems and reduce the burden of on-chain transactions. The main functions of OP_CAT after implementation include:

Multisig Optimization: In multisig scenarios, OP_CAT can help users merge multiple signatures and combine them into a single data block, reducing the number of signatures that need to be submitted. This not only saves on-chain space, but also reduces transaction fees. Multisig is very important in Bitcoin's payment security and shared account management, especially in applications such as the Lightning Network, and OP_CAT can make this process more efficient.

State Contracts: OP_CAT can also be used for state contracts. This type of contract is a form of smart contract that can maintain state across multiple transactions. Through OP_CAT, developers can stitch together state information from different transactions to implement more complex contract logic on Bitcoin. For example, some complex payment protocols or distributed applications (such as lotteries, gambling, or other complex financial products) need to maintain a certain state between multiple on-chain transactions, which can be achieved through OP_CAT.

Scalability of payment channel network: Another important application scenario of OP_CAT is a payment channel network similar to the Lightning Network. In a payment channel network, users typically make a large number of micropayments off-chain, and only go up-chain for final settlement. The splicing function of OP_CAT allows intermediate transactions in payment channels to be more effectively managed and verified. By splicing different payment requests together, users can perform more complex payment operations without increasing the burden on the chain. In this way, the throughput and efficiency of payment channels can be significantly improved.

With the advancement of the BIP 347 proposal and more developers and researchers joining the exploration of OP_CAT, we have reason to believe that this once-shelved opcode will bring new vitality to the Bitcoin network. Just as the Lightning Network has gone from concept to mainstream, the reactivation of OP_CAT may also become the next key milestone in Bitcoin expansion and payment innovation.